1. Field of the Invention
The present invention relates to a toner used for a method for forming images such as an electrophotographic method, an electrostatic recording method, an electrostatic printing method, and a toner jet method. The present invention also relates to a toner and an image forming apparatus that are suitably used for a heating device employed to an on-demand heating system which uses no oil or uses oil only in a small amount.
2. Description of the Related Art
Due to demands for space and energy savings etc., rigorous pursuits have been made to achieve miniaturization, weight reduction, higher speed, and higher reliability of copiers and printers in recent years. Accordingly, those machines are increasingly constructed from components that are simplified in many aspects. As a result, increasingly higher performance is required of toner, and an improvement in toner performance is desired.
Generally, it is required for toners loaded in full-color copying machines that the respective toners be sufficiently color-mixed during the step of heat- and pressure-fixing a toner image, without impairing improvement in color reproducibility and transparency of images on a sheet for overhead projectors (OHP). As compared with black toner generally used for monochrome copying machines, it is preferred that toner for developing full-color images be mainly composed of a low molecular weight binder resin having sharp melting property.
However, in general, using a sharp melt binder resin tends to pose a problem in hot offset resistance owing to the low self-agglomeration force of the binder resin when the toner melts in the step of heat- and pressure-fixing.
In a general black toner for a monochrome copier, as proposed, for example, in JP 52-03304 B, JP 52-03305 B, and JP 57-52574 A, a relatively high crystallinity wax typified by a polyethylene wax or a polypropylene wax is used as a releasing agent in order to improve hot offset resistance upon fixing.
With toners for developing full color images, when an image is projected using the OHP, the image is degraded in chroma and lightness because a transparency of the image is impaired due to high crystallinity of the releasing agent itself and a difference in refractive index from a material of OHP sheet.
In order to solve the above problem, a toner having a specific storage elastic modulus has been proposed. For example, JP 11-84716 A and JP 8-54750 A propose a toner having a specific storage elastic modulus at 180° C. and 170° C., respectively.
However, the above-mentioned toner is too low in viscosity to be used as a color toner that is required to realize both low-temperature fixability and hot offset resistance and provide favorable fixability and satisfactory color-mixing characteristics when used with heat- and pressure-fixing means using no or only a small amount of oil for preventing hot-offset. In addition, the toner proved to be inadequate also in terms of shelf life under high temperature environments.
Further, JP 11-7151 A and JP 6-59504 A propose a toner having a specific storage elastic modulus G′ at 70 to 120° C. and a toner having a specific loss elastic modulus G″ at 130 to 180° C.
However, the above-mentioned toners proved to be inadequate from the viewpoints of providing sufficient shelf life under high temperature environments, obtaining high quality images with stability in outputting images in large quantity, and attaining chargeability and developability with stability irrespective of the usage environment.
In order to solve the above problems, JP 4-149559 A and JP 4-107467 A propose a method for reducing crystallinity of wax by using a core forming agent in combination with wax. In addition, a method for using wax having a low crystallinity is proposed in JP 4-301853 A and JP 5-61238 A. Further, examples of wax having a relatively high transparency and a low melting point include montan-based wax, the use of montan-based wax is proposed in JP 1-185660 A, JP 1-185661 A, JP 1-185662 A, JP 1-185663A, and JP 1-238672 A.
However, the waxes mentioned above are not fully satisfactory in terms of all of the transparency when used with OHP, the low temperature fixability and the hot offset resistance upon heat- and pressure-fixing.
On the other hand, with regard to the heat- and pressure-fixing means in which oil for preventing hot offset is not used or used only in a small amount, various studies have also been made as to the means for improving the binder resin. As one such means, a composition in which wax is added to a resin having an epoxy group is proposed in JP 1-161261 A, JP 5-346686 A, JP 2000-292985 A, and JP 2001-60018 A.
However, none of the above publications refers to viscoelastic properties of a toner for achieving oilless fixing with a sufficiently wide fixable area when those resins are used. Moreover, as a binder resin particularly preferable as a resin that causes a crosslinking reaction with an epoxy group, none of the above publications proposes a combination with a hybrid resin having a polyester unit and a vinyl-based polymer unit.
Conventionally, in image forming apparatuses using an electrophotographic process, a device using a heated roller system is widely used as a fixing device for heat-fixing an unfixed image (toner image) to a recording material (transferring material sheet, electrofax sheet, electrostatic recording paper, OHP sheet, printing paper, formatted paper etc.).
Recently, however, use of image fixing devices using a film heating system have come into wide use from the viewpoints of quick-start and energy saving, as proposed, for example, in JP 63-313182 A, JP 2-157878 A, JP 4-44075 A, JP 4-204980 A, and the like.
In the film heating system, a heat-resisting film (fixing film) is nipped between a ceramic heater serving as a heating member and a pressurizing roller serving as a pressurizing member to form a nip portion, and a recording material bearing an unfixed toner image thereon is introduced between the film and the pressurizing roller in the nip portion. The recording material is then nipped and conveyed together with the film so that, in the nip portion, the heat of the ceramic heater is applied to the recording material through the film, thus heat- and pressure-fixing the unfixed toner image to a surface of the recording material by the heat and the pressurizing force applied in the nip portion.
Such a fixing device using the film heating system can be constructed as an on-demand type device by use of the ceramic heater and a low heat-capacity member serving as the film. Accordingly, the ceramic heater may be turned on by electricity to heat to a predetermined fixing temperature only when carrying out image formation, with the result that a waiting time from the power-on of the image forming apparatus until the image forming apparatus becomes ready to carry out image formation is short (quick-start property), making it possible to considerably restrain the power consumption during stand-by (energy saving property).
From the viewpoints of quick-start and energy saving, adopted as a method different from the above-described method is use of a fixing device that employs an induction heating system utilizing high-frequency induction as a heating source, as proposed in JP 51-109739 U and JP 59-33787 A.
The induction-heating fixing device mentioned above has a coil arranged concentrically therewith in the interior of a hollow fixing roller made of a metallic conductor. An induction eddy current is generated in the fixing roller by means of high-frequency magnetic fields generated by flowing a high-frequency current through the coil, and the fixing roller itself is subjected to Joule heating due to a skin resistance of the fixing roller itself. Electricity/heat conversion efficiency is remarkably improved with the fixing device using the induction heating system, thus making it possible to reduce the warm-up time.
In addition, using a core (magnetic field blocking member) consisting of a magnetic material in combination with the coil allows the high-frequency magnetic fields to be generated in an efficient manner. In particular, when a core having a T-shaped cross section is used, a quantity of heat required of the fixing device can be generated with low power consumption due to efficient concentration of the high-frequency magnetic fields and a magnetic field blocking effect of blocking propagation of the magnetic fields to portions other than a heat generation portion.
However, the prior art techniques described above pose the following problems. In the case of the above-described fixing device using the induction heating system, to make full use of its advantage of reducing the time required until a temperature of the fixing roller surface reaches a temperature suitable for fixing upon startup of the fixing device, it is preferable to make the heat quantity of the fixing roller as low as possible. However, when a thin fixing roller is used for the above reason, it is difficult to set the pressurizing force applied in the nip portion to be high due to the problem of rigidity of the fixing member, which in turn makes it difficult to set a low fixing temperature.
In addition, in the above-mentioned case, heat is not easily transferred in the rotation axis direction of the fixing roller. Thus, for instance, when sheets of paper having a small size are continuously fed, a large difference in the temperature of the fixing roller is liable to develop between a sheet feeding portion and a non-sheet feeding portion. When the sheet heating portion of the fixing roller is adjusted in temperature at this time, a temperature of the non-sheet feeding portion may largely exceed a temperature suitable for fixing, with the result that toner is easily offset to the fixing roller surface in the non-sheet feeding portion or paper clogging easily occurs to tie the paper around the fixing roller.
Further, the high-frequency magnetic fields generated by flowing a high-frequency current through the coil is prone to the problem of scattering upon fixing, because the magnetic fields slightly leak out from the magnetic field blocking member to disturb an unfixed magnetic toner image on the recording material before entering the fixing device. Although these problems can be solved by increasing a transfer current during the transfer process to increase the electrostatic force between the toner and the recording material, this causes a discharge current under a high electric field environment to reach even a photosensitive member, thus damaging a surface of the photosensitive member to reduce the life of the photosensitive member.
Further, in a system using the induction heating type fixing device, as mentioned above, a part of the magnetic material separated from the toner is liable to disturb an unfixed image due to the high-frequency magnetic fields leaking out from the fixing device.
Increasingly sophisticated performance is required of toner applied to the on-demand type fixing system using low heat capacity members as the ceramic heater and the film or the fixing system using the induction heating described above as compared with other systems. Thus, there is a demand for a toner with which fixing can be effected even under a low pressure and a low temperature.
The need for the low-temperature fixability of toner is particularly high in the case of a fixing arrangement in which a large pressure is not applied during the fixing process and a releasing agent is separated by fusing onto the toner surface and fixed; when the releasing agent is not present near the toner surface, releasability from the fixing member is not sufficiently exhibited and the fixability thus deteriorates. Currently, colorization is achieved in the art in such a way that a color is expressed by mixing of multiple colors. Since a large amount of toner needs to be fixed at once, effective use of resin and wax that are advantageous in terms of fixability is critical.
For the above reason, there is desired a color toner which is capable of achieving both low-temperature fixability and offset prevention when used for the above-described heat- and pressure-fixing means in which oil for preventing hot offset is not used or used only in a small amount, and which is also excellent in terms of transparency of a fixed image.